In a system where data is transmitted from a source device to a destination device via one or more buses, transmission mode selecting circuitry is provided to select one of a first transmission mode and a second transmission mode for the data in response to a mode selecting signal that indicates a latency requirement of the destination device. When data is sent using the second mode there is a lower latency between the destination device receiving the data and being able to process the data than when the first transmission mode is used.